Activated carbon for odor control and method for making same

ABSTRACT

An activated carbon-metal oxide matrix is disclosed. The activated carbon-metal oxide matrix may by obtained by a method including the steps of: preoxidizing a carbon material, grinding the preoxidized carbon material; mixing the ground preoxidized material with a metal oxide to form a carbon mixture; extruding the carbon mixture; carbonizing and activating the extrudate. The activated carbon-metal oxide matrix may be used to remove odorous compounds, acidic gases, and volatile organic compounds from a gas.

This application claims priority to U.S. Provisional Application Ser.No. 60/254,900 filed Dec. 11, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to an activated carbon for odor controland method for making same, and more particularly, to an activatedcarbon-metal oxide matrix to control odor in a gaseous stream, andmethod of making same.

2. Description of the Related Art

Activated carbons have long been known for their capacity to sorb odors.Activated carbons capture substances generally through physicalsorption, chemical sorption and catalytic reaction. It is well knownthat the presence of metals in activated carbon can enhance theefficiency and selectivity of the activated carbon in sorptive orfiltering applications. Methods for producing porous structuralmaterials containing adsorbent particles of activated carbon and metalsor metal oxides are conventionally known.

Activated carbon impregnated with metals are typically formed bydispersing activated carbon powders in a solution of a metal salt. Thepowder is filtered out, dried, and heated to decompose the salt to thedesired metal or metal oxide catalyst. Multiple impregnations areusually required to obtain the desired quantity of catalyst on theactivated carbon.

Another technique for making activated carbon supported catalystsinvolves depositing a catalyst metal precursor with high vapor pressureonto a carbon surface. Other methods are known to include extrudingactivated carbon particles with metal or metal oxide particles and abinder.

Siren, in U.S. Pat. No. 4,242,226, discloses an activated carbon matrixfilter material having a metal uniformly dispersed therein. The matrixis obtained by chemically reacting cations that comprise the metal withanion groups chemically bound to a polyhexose derivative. The reactionproduct is separated, pyrolysed and activated.

Tachibana, in U.S. Pat. No. 4,970,189, discloses fine metal particlesdispersed in a carbonaceous mixture. The carbonaceous mixture may beobtained by mixing metal oxide particles with an organic substance andcarbonizing the mixture in a non-oxidizing atmosphere to convert theorganic substance into a porous carbonaceous body and to convert themetal oxide particles into elemental metal particles dispersed in thecarbonaceous body. The metal oxide particles may be coated with ananionic surfactant to provide homogeneity in dispersion of the metaloxide in the organic substance.

Gadkaree et al., in U.S. Pat. No. 5,488,023, disclose a method formaking an activated carbon supported catalyst comprising combining acarbon precursor and a catalyst precursor, curing the carbon precursorif necessary, carbonizing the carbon precursor, and activating thecarbon. The activated carbon supported catalyst can take the form of acoating on a substrate, a powder, or a monolithic body.

Other examples of activated carbons and metal oxides include: U.S. Pat.No. 4,482,641 to Wennerberg; U.S. Pat. No. 4,831,003 to Lang et al.;U.S. Pat. No. 5,948,398 to Hanamoto et al., and U.S. Pat. No. 5,997,829to Sekine et al.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed to an activatedcarbon matrix having between about 3% and about 15% by weight of a metaloxide uniformly dispersed therein.

Another embodiment is directed to a process for preparing a media forfiltering gaseous substances. The process includes preoxidizing a carbonmaterial; grinding the preoxidized carbon; combining the groundpreoxidized carbon and a metal oxide to form a carbon mixture; extrudingthe carbon mixture; and carbonizing and activating the extrudate.

Another embodiment is directed to a method of forming an activatedcarbon-metal oxide matrix including: preoxidizing a carbon material;grinding the preoxidized carbon to form a ground carbon; combining thepowder, coal tar pitch, and the metal oxide to from a paste; extrudingthe paste; and carbonizing and activating the extrudate.

Another embodiment is directed to a method for removing odors from agaseous stream comprising: forming an activated carbon-metal oxidematrix, wherein the matrix has a hydrogen sulfide breakthrough capacitygreater than about 0.3 gH₂S/ccC; contacting the stream with the matrix;sorbing the odorous compound on the matrix; and removing the stream fromthe matrix.

Another embodiment is directed to a method for reducing concentrationsof odorous compounds in a gaseous stream including: contacting thegaseous stream with an activated carbon material comprising about 3 % toabout 15 % of a metal oxide; sorbing the odorous compounds on theactivated carbon material to produce a product stream; and removing theproduct stream from the activated carbon material.

Another embodiment is directed to a method for reducing a concentrationof sulfides present in a gaseous discharge from a waste water treatmentsystem including: providing a gaseous discharge including a volatileorganic compound and a sulfide; contacting the gaseous discharge with anactivated carbon-metal oxide matrix; sorbing the sulfide on the matrixto produce a product stream having a sulfide concentration less thanabout 0.1 ppm; and removing the product stream from the activatedcarbon-metal oxide matrix.

Another embodiment discloses a metal oxide-carrying activated carbon forremoving hydrogen sulfide from a gas including an activated carbon-metaloxide matrix obtained by mixing about 3 % to about 15 % by weight of ametal oxide; carbonizing and activating the matrix.

DETAILED DESCRIPTION

The present invention provides an activated carbon-metal oxide matrixand methods of making and using same. Activated carbon is a porousmaterial characterized by a high carbon content and a large surfacearea, and is typically a mixture of amorphous carbon and graphitecrystals, rather than an homogeneous, well defined material. The term“activated carbon” generally refers to a black, solid carbonaceousmaterial, such as charcoal, bone charcoal, sugar charcoal, carbonproduced from oil products, coconut carbon, and the like, that remainsafter the decomposition of organic material by pyrolysis, and undergoesan activating process, during or after the pyrolysis. Activation istypically done by known methods such as exposing the structure to anoxidizing agent such as steam, carbon dioxide, metal chloride (e.g.,zinc chloride), phospohoric acid, or potassium sulfide, at hightemperatures. Temperatures sufficient for activation generally rangefrom about 800° C. to about 1000° C. (1450° F. to 1850° F.). Activationcreates a high surface area and in turn imparts high adsorptivecapability to the structure.

The activated carbon-metal oxide matrix according to the presentinvention, may be prepared, in general, by preoxidizing a carbonmaterial; grinding the preoxidized carbon material; combining the groundpreoxidized carbon material with a metal oxide to form a carbon mixture;extruding the carbon mixture to form an extrudate; carbonizing theextrudate to form a carbonaceous mixture; and activating thecarbonaceous mixture. The term “matrix” is defined as that which givesorigin or form to a thing or which serves to enclose it. As used herein,the phrase “activated carbon-metal oxide matrix” refers to an activatedcarbon matrix having a metal oxide uniformly dispersed therein.

Any carbon material may be used in the present invention, so long as itresults in a porous carbon material when heated in a non oxidizingcondition. For example, carbon materials usable in the present inventioninclude: charcoal, coconut shell, bone charcoal, sugar charcoal, coaland other conventional carbon materials. The carbon material may bycrushed prior to preoxidation. The carbon material may be ground to apowder. As used herein, the term “powder” is defined as a loose groupingor aggregation of solid particles having a diameter smaller than about 1mm. Alternatively, the carbon may be ground to granules. As used herein,the term “granule” is defined as a loose grouping or aggregation ofsolid particles having a diameter from about 1 mm to about 4 mm. In apreferred embodiment, the carbon material is ground to a granular sizeof about 1 mm to about 2 mm. The ground carbon material is subjected topreoxidation in air at a low temperature, for example about 600° F.

Any metal oxide that enhances the sorptive capacity of activated carbonsmay be used in the present invention. As used herein, the term “sorb” isdefined as the capture of substances generally through physicalsorption, chemical sorption and catalytic reaction. Metal oxides usablein the present invention include metal oxides selected from the groupconsisting of the oxides of Ca, Mg, Ba, Be, Sr, Sc, Y, La, LanthanideSeries, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, andcombinations thereof. In one embodiment, the metal oxide is selectedfrom the oxides of Mg, Ca, and Ba. In a preferred embodiment, the metaloxide is magnesium oxide. The metal oxide may be in any form, such as,for example, granules or powder. The metal oxide in powder form may beof any size and have any size distribution. In a preferred embodiment,the metal oxide powder is about 325 mesh, and more preferably about 200mesh or finer.

The carbon material and metal oxide are mixed to form a carbon mixture.Generally, about 3% to about 15% by weight of the metal oxide is mixedwith the carbon material. In one embodiment, about 5% to about 10% byweight of the metal oxide is mixed with the carbon material. In apreferred embodiment, the carbon mixture comprises about 5% by weight ofthe metal oxide.

In one embodiment, the carbon material and the metal oxide may be mixedin the presence of a binder and, if necessary, a solvent, as is know inthe art to form an extrudable paste. In another embodiment, the carbonmaterial and the metal oxide may be combined to form a carbon mixtureand further ground to a powder before being mixed in the presence of abinder and, if necessary, a solvent to form the extrudable paste. Thecarbon mixture may be ground in a pendulum type-4 ring roll pulverizerutilizing centrifugal force to pass the carbon mixture through a mesh,as is know in the art. In a preferred embodiment, the carbon mixture isground and, if necessary, reground so that approximately 95% of thecarbon mixture passes through a 200 mesh.

The binder may be any known material capable of forming a paste with thecarbon material and metal oxide. For example, the binder may bemolasses, avicel, soft pitch, coal tar, coal tar pitch, and combinationsthereof. In a preferred embodiment, the binder is about 40% coal tar andabout 60% coal tar pitch. The solvent may be any suitable liquid capableof forming an extrudable paste with the carbon material, metal oxide,and binder. For example, the solvent may be water or an organic solvent.In a preferred embodiment, the solvent is water.

The carbon mixture is extruded to form an extrudate capable of beingcarbonized. Extruders, such as high pressure hydraulic extruders, areknown in the art. The extrudate may be of any suitable shape, such as,for example, strands and ribbons. In a preferred embodiment, the carbonmixture is extruded into strands, about 6 mm to about 8 mm long, havinga diameter of about 4 mm. In one embodiment, the extruded carbon mixtureis re-extruded prior to further processing. The extrudate may be allowedto cool to ambient temperature.

The extrudate is carbonized at a temperature and a period of timesufficient to convert the carbon material into a porous carbonaceousmixture. Carbonization is generally performed in the absence of air at atemperature of about 1000° F. If desired, the carbonaceous mixture mayalso be crushed to yield a fine granular product. The carbonaceousmixture is then activated according to known procedures, for example, inthe presence of steam at about 1600° F. to about 1700° F. If desired,the activated carbonaceous mixture may be further treated to obtain thedesired physical characteristics. For example, the finished product maybe screened according to particle size distribution.

Although not being limited to any particular theory, it is believed thatthe metal oxide is highly dispersed throughout the activated carbon andtherefore, does not occupy and reduce the overall pore volume of theactivated carbon. Moreover, addition of the metal oxide in the describedmanner apparently increases the macroporosity and total pore volume ofthe activated carbon.

The activated carbon-metal oxide matrix may be used to sorb odors from awide variety of sources, including: municipal, industrial andresidential sources. For example, the activated carbon-metal oxidematrix of the invention is suitable for sorbing odorous compoundstypical of chemical processes found in sewage treatment plants,refineries, and pulp and paper mills. The activated carbon-metal oxidematrix may also be used to remove odorous compounds from a gas orgaseous stream containing volatile organic compounds, such as, forexample aldehydes and ketones, and/or acidic gasses such as, forexample, butyric acid, hydrogen chloride and sulfur dioxide.

Typical gases that may be purified by contact with the activatedcarbon-metal oxide matrix include, but are not limited to, air, nitrogenand carbon dioxide. Moisture may also be present in the gas so long asit does not condense on the activated carbon-metal oxide matrix. In oneembodiment, the gas has a moisture content of about 60% to about 95% RH.The gas to be purified may also contain oxygen. For example, theactivated carbon-metal oxide matrix of the invention typically oxidizeshydrogen sulfide in the following exothermic reaction.2H₂S+O₂→2H₂O+2SThe activated carbon/metal oxide matrix reduces hydrogen sulfideconcentrations to below odor threshold levels by catalyticaly oxidizingthe hydrogen sulfide to elemental sulfur.

Typical industrial uses may include packing a bed or column with theactivated carbon-metal oxide matrix of the present invention. Forexample, packed beds used in sewage treatment facilities range fromabout 3 feet to about 12 feet in diameter, and about 4 feet to about 6feet in depth with a typical gas velocity through the bed or about 20fpm to about 80 fpm. In a preferred embodiment, the gas velocity isabout 60 fpm. The activated carbon-metal oxide packed bed may beoperated at any pressure to meet throughput and at any temperature belowthe ignition temperature of carbon.

Sewage treatment plants produce sewage gas containing hydrogen sulfideand other organic sulfides that cause it to be malodorous. In addition,most chemical compounds that cause odors in sewage gas are toxic andcorrosive. Examples of sulfur-containing substances known to cause theodor in sewage gas, are, allyl mercaptan, amyl mercaptan, benzylmercaptan, croytl mercaptan, dimethyl sulfide, ethyl mercaptan, hydrogensulfide, and sulfur dioxide, among others. The activated carbon-metaloxide matrix efficiently oxidizes mercaptans to their respectivedisulfides making them more adsorbable.

Hydrogen sulfide, generally the major component of sewage gas, presentat relatively high concentrations, is used as a measure of the odorintensity and corrosiveness of sewage gas. In addition to causing anintense odor associated with rotten eggs, hydrogen sulfide may be quitehazardous, causing physiological effects. A hydrogen sulfideconcentration of about 0.1 ppm of sewage gas can be detected by thehuman nose, which although unpleasant, may be relatively harmless.However, as the concentration of hydrogen sulfide increases, variousphysical effects to exposure may be, for example, headache, nausea, andthroat and eye irritation. At a hydrogen sulfide concentration of about500 ppm of sewage gas, life threatening effects will occur, such aspulmonary edema, nervous system stimulation and apnea. Exposure to ahydrogen sulfide concentration of about 1,000 ppm to about 2,000 ppm ofsewage gas may result in respiratory collapse, paralysis, and death.

The ability of an activated carbon to sorb hydrogen sulfide is reportedin grams of hydrogen sulfide adsorbed per cubic centimeter of carbon,also known as the hydrogen sulfide breakthrough capacity. The hydrogensulfide breakthrough capacity is determined by passing a moist (about85% RH) stream of air containing 1 vol. % hydrogen sulfide through aone-inch diameter tube with a 9-inch deep bed of closely packed carbonat a rate of 1450 cc/min. The stream is monitored to a 50 ppmv hydrogensulfide breakthrough. The activated carbon-metal oxide matrix has aminimum hydrogen sulfide breakthrough capacity of about 0.3 gH₂S/ccC asillustrated in the following Examples.

EXAMPLES

The invention may be further understood with reference to the followingexamples, which are intended to serve as illustrations only, and not aslimitations of the present invention as defined in the claims herein.

Example I

The activated carbon-metal oxide matrix was formed by first crushingbituminous coal and preoxidizing the coal in air at approximately 600°F. The preoxidized coal was ground to a powder and mixed with about 6%magnesium oxide powder. The carbon mixture of preoxidized coal andmagnesium oxide was mixed with coal tar pitch and water; extruded intotypically 4 mm diameter strands; and carbonized in the absence of air atabout 1000° F. The carbonaceous mixture was activated in the presence ofsteam at about 1700° F. The resulting activated carbon-metal oxidematrix was tested for hydrogen sulfide breakthrough. In separate tests,the activated carbon-metal oxide matrix has a hydrogen sulfidebreakthrough capacity of: 0.30, 0.46, 0.54, and 0.65 gH₂S/ccC,respectively.

The hydrogen sulfide breakthrough capacity was also determined forseveral commercially available activated carbons. One such carbon,UOCH—KP® activated carbon impregnated with KOH, available from U.S.Filter Corporation (Los Angeles, Calif.) has a hydrogen sulfidebreakthrough capacity of 0.14, 0.18, and 0.17 in separate tests.Similarly, UOCH—KP® type carbon impregnated with NaOH instead of KOH,also available from U.S. Filter Corporation, has a hydrogen sulfidebreakthrough capacity of 0.18 gH₂S/ccC. Another such carbon, Centaur®4×6, available from Calgon Carbon Corporation (Pittsburgh, Pa.), has ahydrogen sulfide breakthrough capacity of 0.09 gH₂S/ccC.

The activated carbon-metal oxide matrix of the invention has a hydrogensulfide breakthrough capacity 3-5.4 times that of commercially availableimpregnated activated carbons. Because the activated carbon-metal oxidematrix has a greater capacity to sorb hydrogen sulfide than commerciallyavailable impregnated activated carbons, filter beds comprising theactivated carbon-metal oxide matrix may be changed less frequently.Moreover, the activated carbon-metal oxide matrix effectively oxidizeshydrogen sulfide to elemental sulfur with minimal conversion to sulfate(sulfuric acid). Because of this, the pH of the matrix does not changesignificantly with use. Therefore, spent activated carbon-metal oxidematrix is safer to handle than spent impregnated activated carbons, thattypically become very acidic. In addition, the activated carbon-metaloxide matrix has an ignition temperature similar to that for virginactivated carbons, i.e. about 450° C. (842° F.), in contrast to the lowignition temperature associated with impregnated activated carbons, i.e.about 150° C. (302° F.). As a result, the activated carbon-metal oxidematrix is safer to handle than the impregnated activated carbon.

Example II

An activated carbon-metal oxide matrix was formed according to theprocess of Example I. The hydrogen sulfide breakthrough capacity of theactivated carbon-metal oxide matrix measured in a gas stream fullysaturated with xylene was 0.26 gH₂S/ccC. The hydrogen sulfidebreakthrough capacity of UOCH—KP® carbon in a gas stream fully saturatedwith xylene was 0.04 gH₂S/ccC. Presence of xylene in the stream reducesthe average hydrogen sulfide breakthrough capacity of the impregnatedcarbon by approximately 75%, and of the matrix by approximately 47%. Thematrix is, therefore, less sensitive to organics in a stream thancommercially available impregnated activated carbons.

The above description and examples are meant to be taken as exemplaryonly, of preferred embodiments of the invention. As such, the inventioncan be practiced according to other techniques and equivalents thereof.

1-4. (canceled)
 5. A process for preparing a media for filtering gaseoussubstances, comprising: preoxidizing a carbon material to formpreoxidized carbon; grinding the preoxidized carbon; combining theground preoxidized carbon and a metal oxide to form a carbon mixture;extruding the carbon mixture to form an extrudate; carbonizing theextrudate to form a porous carbonaceous mixture; and activating theporous carbonaceous mixture.
 6. The process of claim 5, wherein grindingthe preoxidized carbon forms granules.
 7. The process of claim 6,further comprising grinding the carbon mixture prior to extruding thecarbon mixture.
 8. The process of claim 7, wherein the carbon mixture iscombined with a binder prior to extruding the carbon mixture.
 9. Theprocess of claim 8, wherein the carbon mixture and binder is combinedwith a solvent prior to extruding the carbon mixture
 10. The process ofclaim 5, wherein grinding the preoxidized carbon forms a powder.
 11. Theprocess of claim 10, wherein the ground preoxidized carbon and the metaloxide are combined in the presence of a binder.
 12. The process of claim11, wherein the binder is coal tar pitch.
 13. The process of claim 12,wherein the ground preoxidized carbon, the metal oxide, and the coal tarpitch are combined in the presence of water to form a carbon mixture.14. The process of claim 13, wherein the carbon mixture formed is apaste.
 15. The process of claim 5, wherein the coal is peroxidized inair at approximately 600° F.
 16. The process of claim 5, wherein a metaloxide at about 3% to about 15% by weight is combined with groundpreoxidized carbon.
 17. The process of claim 16, wherein a metal oxideat about 5% to about 10% by weight is combined with the groundpreoxidized carbon.
 18. The process of claim 15, wherein the extrudateis carbonized in the absence of air at approximately 1000° F.
 19. Theprocess of claim 18, wherein the carbonaceous mixture is activated withsteam between about 1600° F. and about 1700° F.
 20. The process of claim5, further comprising crushing the carbon material before preoxidizingthe coal.
 21. The process of claim 5, further comprising crushing thecarbonaceous mixture prior to activating the carbonaceous mixture. 22.The process of claim 5, wherein the carbon material to be preoxidized isselected from the group consisting of: coconut shell and coal.
 23. Theprocess of claim 22, wherein the coal is bituminous low ash coal. 24.The process of claim 5, wherein the metal oxide is selected from thegroup consisting of the oxides of Ca, Mg, Ba, and combinations thereof.25. The process of claim 24, wherein the metal oxide is magnesium oxide.26. A method of forming an activated carbon-metal oxide matrixcomprising: preoxidizing a carbon material to form a preoxidized carbon;grinding the preoxidized carbon to form ground carbon; combining theground carbon, the metal oxide and coal tar pitch to form a paste;extruding the paste to form an extrudate; carbonizing the extrudate toform a carbonaceous mixture; and activating the carbonaceous mixturewith steam.
 27. The method of claim 26, wherein the ground carbon is apowder.
 28. The method of claim 26, wherein the ground carbon isgranules.
 29. The method of claim 28, wherein the act of combining theground carbon, metal oxide and coal tar pitch includes grinding metaloxide combined with the ground carbon prior to combining with the coaltar pitch.
 30. The method of claim 26, wherein the metal oxide iscombined at about 3% to about 15% by weight.
 31. The method of claim 30,wherein the metal oxide is combined at about 5% to about 10% by weight.32. The method of claim 26, wherein the carbon material is preoxidizedin air at approximately 600° F.
 33. The method of claim 32, wherein theextrudate is carbonized in the absence of air at approximately 1000° F.34. The method of claim 33, wherein the carbonaceous mixture isactivated with steam between about 1600° F. to about 1700° F.
 35. Themethod of claim 26, wherein the carbon material is coal.
 36. The methodof claim
 26. wherein the metal oxide is selected from the groupconsisting of the oxides of Ca, Mg, Ba, and combinations thereof. 37.The method of claim 36, wherein the metal oxide is magnesium oxide.38-65. (canceled)